The catalyst systems useful in the practice of this invention are known broadly as olefin metathesis catalysts. The olefin metathesis reaction is a general reaction of olefins, both cyclic and acyclic, and is known to proceed by the catalyzed cleavage of carbon-to-carbon double bonds, and the subsequent recombination of the resulting fragments to form new olefinic species: ##STR1##
When cycloolefins react in the presence of an olefin metathesis catalyst, ring cleavage occurs, and high molecular weight polymers result. Thus, cyclopentene yields the linear polymer polypentenamer: ##STR2##
The structure of this cyclopentene polymer may also be represented by the equivalent formula: EQU --CH.dbd.CHCH.sub.2 CH.sub.2 CH.sub.2 --.sub.n
As a further example, when dicyclopentadiene reacts, only one of the double bonds normally undergoes the metathesis reaction and so polymerization occurs to give a predominantly linear polymer: ##STR3##
The structure of the dicyclopentadiene polymer may also be represented by the equivalent formula: ##STR4##
A wide variety of catalysts suitable for the metathesis of acyclic olefins and for the homopolymerization of cycloolefins are known in the art, but considerable difficulties have been encountered in attempts to prepare rubbery, soluble copolymers from dicyclopentadiene and cyclopentene which contain a substantial proportion of dicyclopentadiene, i.e., greater than about 10% by weight of dicyclopentadiene. Thus, in U.S. Pat. No. 3,598,796 there is disclosed a process for making rubbery homopolymers of cyclopentene. However, when a mixture of cyclopentene and dicyclopentadiene containing 20% by volume of dicyclopentadiene was polymerized in a solvent, the product was found to be non-elastomeric and only partially soluble.
It has been found that dicyclopentadiene generally has a great tendency to homopolymerize in the presence of other cycloolefins, and that the resulting blocky segments of non-rubbery homopolymers greatly interfere with the desired elastomeric properties of the intended copolymers, causing them to become stiff and inelastic and unsuitable for use in articles where rubbery qualities are required. These copolymers are further generally characterized as being opaque or translucent rather than transparent, and are only poorly soluble in customary rubber solvents such as benzene, toluene, hexane, cyclohexane and the like.
The terms "rubbery" and "non-rubbery", as used herein, describe in a qualitative way, the combination of properties of a solid material which result from its inherent hardness, elasticity, and resilience, and are not expressed easily in quantitative terms. However, one can measure and compare the stiffness, or hardness, of a material as one measure of rubberiness, since conventional rubbers are characterized as being relatively soft at temperatures of about 25.degree. C.
Stiffness can be expressed as the Young's modulus of a material. Typical values for the Young's modulus of familiar elastomers may be found in "Polymers Handbook, Section Edition", J. Brandrup and E. H. Immergut, Ed., John Wiley and Sons, N.Y., 1975, pages V-7 et seq. Thus, common elastomers such as polyisoprene, butyl rubber, SBR (styrene-butadiene rubber) and polychloroprene are stated to have values of Young's modulus ranging from about 1.0 to about 1.6 mega pascal (MPa) in the absence of fillers such as carbon black or plasticizers such as processing oils. Plastics such as polyethylene, polypropylene and polystyrene, on the other hand, all have much higher Young's modulus values and are characteristically quite stiff and non-rubbery.
In order to overcome problems associated with the copolymerization of dicyclopentadiene and cyclopentene, certain procedures have been proposed. Thus, in U.S. Pat. No. 3,707,520 there is described a two-stage process whereby, in the first stage, cyclopentene is polymerized to at least 40% conversion, followed by the gradual introduction of dicyclopentadiene during the second stage of the polymerization. However, if the polymerization of cyclopentene is not carried to at least 40% conversion in the first stage, the final product is found to be unsatisfactory.
Similarly, U.S. Pat. No. 3,941,757 describes a two-stage process wherein the homopolymerization of cyclopentene is first initiated, and thereafter a solution containing a polycyclic olefin such as dicyclopentadiene and also containing a tungsten or molybdenum compound is very gradually introduced into the polymerizing cyclopentene solution. It is essential that the transition metal compound be present in the solution of the polycyclic olefin when it is added to the polymerizing mass. If the transition metal compound is not present in this solution prior to its introduction into the polymerization mixture, then only a low yield of polymer having excessive stiffness resulted.
In order to avoid the necessity of using a two-stage polymerization process and at the same time overcome the tendency to form a substantial amount of insoluble polymer, an alternative method has been proposed for the preparation of cyclopentene/dicyclopentadiene copolymers which contain very little insoluble polymer. Thus, U.S. Pat. No. 4,002,815 describes a polymerization process wherein organoaluminum iodides, or optionally, a combination of a trialkylaluminum compound and elemental iodine, are employed as essential cocatalysts in conjunction with soluble tungsten compounds. Organoaluminum chlorides are unsatisfactory. Additionally, this process requires the presence of from 1 to 30 mole percent of an acyclic olefin, relative to the total amount of monomers, in order to avoid the formation of insoluble products. However, if less than 1 mole percent of acyclic olefin were used, yields were poor or the products contained substantial amounts of insoluble polymer.
Also, see U.S. Pat. No. 3,933,778.
Further, each of the above processes have been demonstrated to be effective when aromatic solvents such as benzene, toluene or chlorobenzene are used. However, yields and rates of polymerization are much inferior when aliphatic or cycloaliphatic solvents are employed and consequently undesirably high catalyst concentrations and long reaction times become necessitated. Thus, these processes are unattactive for industrial applications where aliphatic or cycloaliphatic solvents need be employed.
It is, therefore, an object of the present invention to provide an efficient method for the preparation of rubbery, essentially gel-free copolymers of cyclopentene and dicyclopentadiene, wherein aliphatic or cycloaliphatic solvents may be effectively employed as well as aromatic solvents.